The present invention relates to a double image recording method, and more particularly to a method of recording an image on a recording medium at one surface thereof in overlapping relation to an existing image previously recorded thereon or at the reverse side of the recorded surface.
Some recording apparatus such as a laser printer or a digital copier employing a CCD (charge-coupled device) operate according to the electrophotographic process. In such recording apparatus, generally, a developed image is transferred to a recording medium such as a sheet of paper, and then fixed to the recording medium. The image is fixed by a fixing device of the heating type wherein the image is thermally fused to the sheet, which is thereafter discharged as a copy out of the recording apparatus.
In recent years, images are copied in various different ways. For example, different images are copied on the opposite surfaces of one sheet of paper. According to another example, an image is formed on one surface of a sheet of paper, and another image is inset in a space in the previous image. To perform such a copying mode, a sheet of paper to which an image has been fixed is introduced again into the recording apparatus in which another image is recorded on the sheet.
When an image is fixed with heat to a recording medium such as a sheet of paper, the recording medium is caused to shrink because the moisture thereof is evaporated by the heat. Therefore, another image subsequently recorded on the sheet of paper in overlapping relation to the previous image may be shifted out of registry therewith since they are recorded at different magnification ratios.
It is known that such a phenomenon arises out of the relationship between time for which the sheet is left after an image has been fixed thereto and a sheet shrinkage factor, as indicated by the solid-line curve in FIG. 5 of the accompanying drawings.
FIG. 6 shows a process for recording images on both surfaces of a single sheet of paper. After a first image has been recorded on one surface of the sheet at (1), it is fixed with heat and, immediately thereafter, the sheet shrinks because the moisture content thereof is reduced at (2). Then, a second image is recorded on the other surface of the sheet at (3), and thereafter fixed thereto. While the sheet is being left, the sheet restores its original size by absorbing humidity at (4). However, the first and second images are of different sizes, the difference being commensurate with the shrinkage and expansion of the sheet. During the recording process, the sheet shrinks and expands as indicated by the arrows in FIG. 3.